1. Field of the Invention
The present invention relates to a DC/DC converter, and more particularly, to a DC/DC converter with spread spectrum switching signals for turning on and off switches.
2. Description of the Prior Art
Please refer to FIG. 1. FIG. 1 is a diagram illustrating a conventional DC/DC converter 100. As shown in FIG. 1, the DC/DC converter 100 comprises an inductor L1, a switch Q1, a diode D1, a capacitor C1, two resistors R1 and R2, and switch controller 110. The DC/DC converter 100 receives a first DC power source (of a voltage V1), converts the first DC power source into a second DC power source (of a voltage V2), and outputs the second DC power source. The inductor L1 is coupled to the input end for receiving the first DC power source. The switch Q1 is coupled between the inductor L1 and the ground end. The diode D1 is coupled between the switch Q1 and the capacitor C1. The capacitor C1 is coupled between the output end and the ground end. The resistor R1 is coupled between the output end and the resistor R2. The resistor R2 is coupled between the resistor R1 and the ground end. The switch controller 110 is coupled to the control end of the switch Q1.
The switch controller 110 controls turning on and off the switch Q1. When the switch Q1 turns on, the current is stored in the inductor L1. When the switch Q1 turns off, the stored current in the inductor L1 passes through the diode and charges the capacitor C1. The output voltage V2 is the voltage over the capacitor C1. The switch controller 110 controls the turning-on period in each cycle (duty cycle) so as to adjust the output voltage V2.
The resistors R1 and R2 are utilized as voltage dividers. The voltage [R2/(R1+R2)]×V2 is fed back to the switch controller 110 for informing the switch controller 110 of the voltage V2.
The switch controller 110 receives a reference clock signal CLK1, and outputs a control signal S1 to the switch Q1 according to the voltage [R2/(R1+R2)]×V2. The frequency of the control signal S1 is same as the clock signal CLK1 but the duty cycle adjusting the control signal S1 is set according to the voltage [R2/(R1+R2)]×V2 for controlling turning-on/off period of the switch Q1. Thus, the voltage V2 is controlled. That is, when the duty cycle of the control signal S1 is larger, the turning-on period is longer, and the current stored in the inductor L1 is bigger, and thus, the voltage V2 rises gradually. On the other hand, when the duty cycle of the control signal S1 is lower, the turning-on period is shorter, and the current stored in the inductor L1 is smaller, and thus, the voltage V2 drops gradually. Additionally, the reference clock signal CLK1 is 12 MHz.
However, the conventional switch controller transmits the control signal S1 based on a fixed frequency and an adjustable duty cycle. That is, the control signal S1 generates a base frequency (12 MHz), which is a source of interference in general portable electronic devices, causing bad connections and impacting the convenience of the portable electronic devices.